1. Field of the Invention
The present invention relates to multilayered ceramic boards, methods for fabricating the same, and electronic devices using such multilayered ceramic boards. More particularly, the invention relates to improvements in densification of a multilayered ceramic board fabricated by a so-called xe2x80x9cnon-shrinkage processxe2x80x9d
2. Description of the Related Art
A multilayered ceramic board includes a plurality of ceramic layers which are laminated. Various wiring conductors are provided on the multilayered ceramic board. As the wiring conductors, for example, internal conductive films extending along specific interfaces between the ceramic layers are formed in the multilayered ceramic board; via-hole conductors extending so as to pass through specific ceramic layers are formed; and external conductive films extending on the external surfaces of the multilayered ceramic board are formed.
Multilayered ceramic boards are used for interconnecting semiconductor chip components and other chip components mounted thereon. Wiring conductors as described above serve as the electrical paths for the interconnections.
Passive elements, such as capacitors and inductors, may be built in multilayered ceramic boards. In such a case, the passive elements are formed by parts of the internal conductive films and via-hole conductors which are the wiring conductors.
For example, multilayered ceramic boards are used as high-frequency LCR composite parts in the field of mobile communication terminal equipment. In the field of computers, multilayered ceramic boards are used as composite parts including active elements, such as semiconductor IC chips, and passive elements, such as capacitors, inductors and resistors, or simply used as semiconductor IC packages.
Multilayered ceramic boards are widely used for forming various electronic components, such as PA module boards, RF diode switches, filters, chip antennas, various package components, and composite devices.
In order to increase the level of functionality, the density and the performance of multilayered ceramic boards, it is effective to dispose wiring conductors as described above at a high density. However, a firing step must be carried out in order to fabricate a multilayered ceramic board. In such a firing step, shrinkage occurs due to sintering of the ceramic, and the shrinkage tends to be nonuniform over the entire multilayered ceramic board, resulting in undesired deformation and strain in wiring conductors. Such deformation and strain occurring in the wiring conductors inhibit an increase in the density of the wiring conductors.
Therefore, use of a so-called xe2x80x9cnon-shrinkage processxe2x80x9d has been proposed, in which shrinkage in the principal surface direction of the multilayered ceramic board is substantially prevented in the firing step when a multilayered ceramic board is fabricated.
In the fabrication method of a multilayered ceramic board using the non-shrinkage process, a low-temperature sinterable ceramic material which can be sintered, for example, at a temperature of about 1,000xc2x0 C. or less is prepared, and simultaneously an inorganic material powder which is not sintered at the sintering temperature of the low-temperature sinterable ceramic material and which inhibits shrinkage is prepared. When a green laminate for forming the predetermined multilayered ceramic board is fabricated, a plurality of green base layers containing the low-temperature sinterable material are laminated, and the green constraining layers containing the inorganic material powder are disposed so as to be in contact with the principal surfaces of specific green base layers. Wiring conductors are also provided on the green base layers.
The green laminate thus obtained is then fired. In the firing step, a substantial shrinkage does not occur in the green constraining layers since the inorganic material contained in the green constraining layers is not substantially sintered. Consequently, the green constraining layers constrain the green base layers, and thereby, although the green base layers substantially shrink only in the thickness direction, shrinkage in the principal surface direction is inhibited. As a result, nonuniform deformation does not easily occur in the multilayered ceramic board which is obtained by firing the green laminate, and thereby, it is possible to prevent undesired deformation and strain in the wiring conductors, enabling an increase in the density of the wiring conductors.
The green base layers contain the low-temperature sinterable ceramic material and a binder. The low-temperature sinterable ceramic material contains a ceramic powder and a glass component. The glass component contained in the low-temperature sinterable ceramic material may be contained as a glass powder from the beginning or a glass substance may be precipitated in the firing step. In some cases, the glass component may precipitate a crystalline substance in at least the final stage of the firing step, and thereby is crystallized.
At any rate, a dense state must be produced in the ceramic layers obtained by firing the green base layers. For that purpose, the ceramic powder contained in the green base layers must be densified, and in order to density the ceramic powder, it is important that the fluidity of the glass component is ensured in the firing step.
However, as described above, since the green base layers are constrained by the green constraining layers so that the green base layers substantially shrink only in the thickness direction, the fluidity of the glass component tends to be inhibited. For this reason, it is important that the fluidity of the glass component is ensured.
When the glass component which precipitates the crystalline substance in at least the final stage of the firing step is contained in the low-temperature sinterable ceramic material, the viscosity of the glass component increases as the crystalline substance is precipitated, and as a result, fluidity of the glass component is lost. Therefore, the densification of the ceramic powder does not easily advance.
Accordingly, objects of the present invention are to provide a method for fabricating a multilayered ceramic board using a so-called xe2x80x9cnon-shrinkage processxe2x80x9d in which dense ceramic layers can be formed, to provide a multilayered ceramic board fabricated by such a method, and to provide an electronic device using the multilayered ceramic board.
In one aspect of the present invention, a method for fabricating a multilayered ceramic board includes a lamination step for forming a green laminate, the green laminate including a plurality of green base layers containing a low-temperature sinterable ceramic material comprising a ceramic powder and a glass component, and a binder; at least one green constraining layer disposed in contact with a principal surface of a specific green base layer, the green constraining layer containing an inorganic material powder which is not sintered at the sintering temperature for the low-temperature sinterable ceramic material; and wiring conductors provided on the green base layers; and a firing step for firing the green laminate at the sintering temperature for the low-temperature sinterable ceramic material. The firing step includes a binder removal step for removing the binder contained in the green base layers and a sintering step for obtaining the sintered state of the low-temperature sinterable ceramic material in which the ceramic powder is densified while the glass component is fluidized in the green base layers. In order to solve the technical problems described above, the rate of temperature increase from the binder removal step to the sintering step is set to be more than about 20xc2x0 C./minute.
In another aspect of the present invention, a glass component contained in a low-temperature sinterable ceramic material in a method for fabricating a multilayered ceramic board, can precipitate a crystalline substance, and in the firing step, the rate of temperature increase is controlled so that the glass component precipitates the crystalline substance after a ceramic powder is densified.
In the method for fabricating a multilayered ceramic board of the present invention, when the green constraining layers are disposed on both ends in the lamination direction of the laminate, the green constraining layers located on both ends in the lamination direction of the laminate may be removed after the firing step is carried out.
The method for fabricating a multilayered ceramic board of the present invention may further include a step of mounting an electronic component on an external surface of the laminate after the firing step is carried out.
In another aspect of the present invention, a multilayered ceramic board is fabricated by the fabrication method described above.
In another aspect of the present invention, an electronic device includes the multilayered ceramic board and a motherboard for fixing the multilayered ceramic board.
As described above, in accordance with the present invention, when a multilayered ceramic board is fabricated, since the co-called xe2x80x9cnon-shrinkage processxe2x80x9d is employed, although fluidity of the glass component contained in the green base layers tends to be inhibited because of the constraint of the green constraining layers, since the rate of temperature increase is set to be more than about 20xc2x0 C./minute, the glass component can be fluidized at a relatively early stage and the fluid state of the glass component can be sufficiently prolonged. As a result, the ceramic powder contained in the green base layers starts to be densified at a relatively early stage, and thereby it is possible to obtain a satisfactorily dense state in the ceramic layers produced by firing the green base layers.
When the glass component contained in the green base layer can precipitate a crystalline substance, the ceramic powder can be thoroughly densified before the crystalline substance is precipitated and the fluidity of the glass component is lost. Consequently, the ceramic layers obtained can be satisfactorily densified.
Accordingly, the multilayered ceramic board fabricated by the fabrication method of the present invention can provide superior electrical characteristics and reliability. Therefore, if an electronic device is fabricated by mounting the multilayered ceramic board on a motherboard, the electrical characteristics and reliability of the electronic device can also be improved.